The invention relates to a stacker wheel as well as a stacking apparatus comprising one or several such stacker wheels for stacking sheet material. The invention furthermore relates to a sheet material processing apparatus comprising such a stacking apparatus, in particular for processing value documents such as for example bank notes. Such sheet material processing apparatuses can accordingly be money depositing and/or dispensing machines, value document processing machines in general and bank note processing machines for checking bank notes in particular.
Stacker wheels of the above-mentioned type possess sheet pockets distributed over the circumference for receiving one or several sheets of the sheet material to be stacked. The sheet material pockets extend in the stacker wheel from radially outside to radially inside along a substantially spiral-shaped course. Therefore, sheet material processing apparatuses that are equipped with such stacker wheels are normally referred to as “spiral pocket stackers”.
Automatic sorting machines in which sheet-shaped objects, for example bank notes, are sorted must be able to process high numbers of items in as short a time as possible, which necessarily leads to high transport speeds. With conventional spiral pocket stackers, which have for example an outside diameter of 220 mm, an increase of the transport speed to more than 7.5 m/s is problematic, which corresponds to a processing of 30 bank notes per second. The problem consists substantially in the fact that upon their processing the bank notes, when running into the stacker wheel, are slowed down to a standstill within a short time on a short way. In particular with new and also with large-format bank notes this leads to the bank notes hitting against the end of the sheet pockets and consequently to damage at the leading bank note edges.
In DE-AS 12 48 561 there is described a spiral pocket stacker, wherein two disks rotating around a joint shaft and having slots extending in spiral-shaped fashion from the outside to the inside are arranged side by side. The slots of the discs that axially lie in congruence form one storage pocket each, in which a sheet is tangentially introduced. With the help of a stripper arranged between the discs, the sheets are discharged from the storage pockets. The circumferential speed of the discs is substantially lower than the transport speed of the sheets, so that the sheet running in frictionally slides with its surface along the outer boundary walls of the spiral-shaped slots. The frictional force arising from the relative motion slows down the sheet. Through the centrifugal force which becomes effective as a result of the diversion into a spiral path, the pressure of the documents against the corresponding boundary walls is further increased, which is why the friction, however dependent on the speed, is intensified. However, the slowing down may be insufficient, even when the stacking apparatus is provided with several spiral disks for increasing the effective friction surface. Primarily more rigid sheets, such as for example new bank notes before they are brought into circulation, bounce with too high speeds against the sheet pocket ends or against the stripper and are reflected therefrom. In so doing, the sheet may prematurely exit from the sheet pocket, which necessarily leads to a failure of the stacking process. If sheets with rather soft, limp quality impinge with too high speeds on the sheet pocket ends or on the stripper, an accordion-like deformation, primarily in the front region of the sheet, is possible. Damage or at least an inaccurate alignment in the subsequent stacking is often the result.
There have been made all sorts of proposals as to how the friction for slowing down the sheets upon their running into the stacker wheel can be increased. In DE 32 32 348 A1 it is proposed to mutually “stagger” the spiral-shaped sheet pockets of stacker wheels arranged side by side on a joint drive shaft. Staggering is understood to mean that the sheet pockets extending in spiral-shaped fashion, upon viewing along the drive shaft, are not congruent to each other but extend differently. The sheets running into the sheet pockets thus go wavy in a direction transverse to the run-in direction. This can be achieved in that individual sheet pockets have a different spiral curvature than other sheet pockets, on the one hand. The same effect can also be achieved, on the other hand, with identical stacker wheels, whose sheet pockets all have the same spiral curvature, when at least one of the stacker wheels is mounted on the drive shaft rotated by a small angular amount relative to the other stacker wheels. On account of the wavy curvature imposed upon the running into the stacker wheels, the pressure of the sheets against the boundary walls of the spiral slots is intensified and, consequently, frictional forces and thus the braking effect increase accordingly.
DE 101 10 103 A1 complains about this, that the imposed waving may lead to slight deformations of the bank notes in the sheet pockets and thus to an inaccurate stacking. In DE 101 10 103 A1 it is instead proposed to adjust the width of the sheet pockets to the type of the sheets to be respectively stacked. For this purpose, the stacker fingers, respectively two neighboring stacker fingers thereof forming a storage pocket, are rotatably mounted at the stacker wheel shaft. By swiveling the stacker fingers in the one direction or the other the distance of the outer tips of the neighboring stacker fingers and consequently the width of the associated sheet pocket is enlarged or diminished. This ultimately again influences the braking force that acts on the sheets running in. Such mechanisms are elaborate in the manufacturing and prone to maintenance.
Instead of this, also separate braking bodies can be used, which are arranged in meshing fashion with the stacker wheels, in order to form an additional braking surface for the sheets running in. In DE 10 2008 000 026 B3 it is for example proposed to configure such a braking body as a rotating braking roller. This type of slowing down requires additional components, which increases the manufacturing effort and maintenance effort.
For a stronger slowing down of the sheets, also stacker wheels having a larger diameter and thus a longer braking distances can be employed. This, however, is disadvantageous due to the increased space requirement.
It is therefore the object of the present invention to create a stacking apparatus for processing sheet material, such as bank notes, wherein, even at high transport speeds, damage to the sheet material to be stacked is avoided.